The recent drastic advancement in technologies of optical designs and manufacturing optics has enabled zoom lenses to be downsized more and enhanced in variable power. For instance, one type of the zoom lenses in the state of the arts have a variable power enhanced design and enable to incorporate an optical stabilizer or anti-vibration feature, which typically comprise four groups of lens pieces, namely, the 1st lens group of positive refractivity, the 2nd lens group of negative refractivity, the 3rd lens group of positive refractivity, and the 4th lens group of positive refractivity arranged in this order on the closer to objects first basis where the power is varied from the wide-angle end to the telephoto end as a result of the 1st and 2nd lens groups being split more, the 2nd and 3rd lens groups coming closer to each other, and the 3rd and 4th lens groups being varied in a distance therebetween. The 3rd lens group has the leading set of the lens pieces of positive refractivity closer to objects and the trailing set of the lens pieces of negative refractivity, and the trailing set alone are moved in position approximately orthogonal to the optical axis so as to compensate for defocus in the imaging plane caused by a tremor of the user's hand(s), which can be corrected by the anti-vibration feature of which requirements are to satisfy the following formulae:3.5<f1/fw<8.0where fw is a focal length of the zoom lens at the wide-angle end, and f1 is the focal length of the 1st lens group (see Patent Document 1 listed below).
Another type of the high variable power zoom lenses are compatible with an APS-C dimensioned image sensor and are dedicated to digital cameras, which also have four groups of lens pieces, namely, the 1st lens group of positive refractivity, the 2nd lens group of negative refractivity, the 3rd lens group of positive refractivity, and the 4th lens group of positive refractivity arranged in this order on the closer to objects first basis where the power is varied from the wide-angle end to the telephoto end as a result of the 1st and 2nd lens groups being split to elongate an aerial distance therebetween, and the 2nd and 3rd lens groups coming closer to each other while simultaneously the 1st, 3rd, and 4th lens groups are moved all together closer to objects. The 2nd lens group are displaced for the focusing of which requirements are to satisfy the following formulae:0.40<fW/fbW<0.55   (1)0.43<β34W/β34T<0.47   (2)40<r5/d5W<100   (3)1.1<f3/f4<2.6   (4)where fW is a focal length of the zoom lens at the wide-angle end, fbW is a back focal length at the wide-angle end, β34W is a composite power of the 3rd and 4th lens groups at the wide-angle end, β34T is the composite power of the 3rd and 4th lens groups at the telephoto end, r5 is a radius of curvature of the surface designated by r5, d5W is a distance between the 1st and 2nd lens groups at the wide-angle end, f3 is the focal length of the 3rd lens group, and f4 is the focal length of the 4th lens group. This type of the zoom lenses are, regardless of their original design concept especially suitable for the APS-C size, able to ensure the same back focusing as those for image dimensions for 3.5 mm film (see Patent Document 2).
List of Patent Documentations                Patent Document 1        
Japanese Patent Unexamined Publication No. 2006-106191                Patent Document 2        
Japanese Patent Unexamined Publication No. 2005-331697
As the consumer demands in the camera market have been trending from the conventional silver film cameras to digital single-lens reflex cameras, contaminants likely attached to optical imaging components such as charge-coupled devices (CCDs) and the like have become a matter of concern because of their adverse effects upon the resultant images. Due to the contaminants onto the CCDs, component lenses must be often replaced, and one of the goals of the newly developed high variable power zoom lenses is to avoid such frequent replacement. Among the significantly advanced group of the improved zoom lenses, none has attained the variable power higher than 15× while some of the remaining less aggressive groups cover as wide as about 75 degrees in field angle at the wide-angle end and are no more than 3 to 4 in F-number at the wide-angle end.
In general, as the variable power is raised, the lens groups are accordingly displaced more, associated with the increased variations in aberration, which results in compensation for the aberration being more difficult throughout the focal range. To overcome this, the lens groups must have their respective refractivities diminished to correct the aberration, or some of the lens pieces should be shaped to have an aspherical surface(s) for the same purpose. Reducing the refractivity in such manners, however, resultantly necessitates the lens groups to be displaced more for varying the power as much, and this leads to a more complicated cam barrel design as well as an increase in diameter of the zoom lens.
In the zoom lens as described in Patent Document 1, the component lens piece(s) has an aspherical surface(s), and there are three of the lens pieces in the leading set so as not to increase the number of the lens pieces for correcting the aberration. In the event of the power raised higher than 13×, the zoom lens is not able to sufficiently compensate for spherical aberration that would be caused in the 1st and/or 2nd lens groups at the telephoto end. In addition, another problem of an increase in the diameter of a filter arises.
The zoom lens disclosed in Patent Document 2 has the variable power of approximately 7×, and hence, even if the 3rd lens group has its refractivity diminished, it is avoidable that, as a result of varying the power of the zoom lens, the refractivity would never be reduced so much as −1× at which this lens group makes the comprehensive power invariable. Raising the variable power of the zoom lens up to 13× results adversely in the zoom lens being in focus at the varied focal length to cause the 3rd lens group to be diminished in power as low as −1× in some case. Additionally, the 3rd lens group becomes more aberration sensitive, and coping with this kind of troubles brings about disincentive and degrades their productivity.
The present invention is made to overcome the aforementioned problems with the prior art high power zoom lenses, and accordingly, it is an object of the present invention, especially of first two aspects of the invention described later, to provide a high power zoom lens of multi groups of lens pieces that is in focus at the varied focal length never to cause the 3rd lens group to be diminished in power as low as −1×.
It is another object of the present invention, especially of the first two aspects of the invention described later, to provide a high power zoom lens of multi groups of lens pieces where the 3rd lens group has a reduced sensitivity to aberration, which brings about incentive to enhance their productivity.
It is still another object of the present invention, especially of the first two aspects of the invention described later, to provide a high power zoom lens of which filter is effectively downsized in diameter.
It is yet another object of the present invention, especially of a third aspect of the invention described later, to provide a high power zoom lens that facilitates incorporation of an aperture stop controlling mechanism and an optical stabilizer or anti-vibration mechanism by elongating a distance from an aperture stop to an anti-vibration/compensation lens component and a distance from the latter to an image plane, and that is capable of enhancing the variable power up to as high as 18-270 mm in focal length.
It is further another object of the present invention, especially of the third aspect of the invention described later, to provide a high power zoom lens that has optics of a relatively small longitudinal dimension and has a filter of a reduced diameter.